Backpacks typically include a packbag which is designed to carry items of interest to the wearer. The packbag is sometimes carried on a rigid frame. Most backpacks have a waist belt attached to the bottom portion of the packbag that is made to encircle the wearer's torso and is adjustable for a snug fit. Some of the weight of the pack is carried by the shoulder straps, which are also adjustable so that the packbag can be snugged against the wearer's back and hips as desired.
Because these backpacks rely on the shoulder straps to carry some of the load, the packs exert a substantial backwards pull at the shoulders of the wearer, causing back fatigue and strain. Further, the shoulder straps hinder arm and shoulder movement, which limits the enjoyability of wearing the backpack.